The present invention relates to image recorders for recording images on a substrate, such as photographic film, and more particularly to a rearview cathode ray tube (CRT) that is useful in providing images for recordal with improved dynamic range, higher resolution and fewer errors.
CRTs are well known devices for the electronic display of images. For example, television sets use CRTs to display video images.
CRTs are also used for image generation and scanning purposes in connection with the recordal and reading of text and/or graphic images. Such devices are in widespread use because of the ease with which electron beams can be scanned and converted into visible energy on phosphor screens.
One type of image recorder that utilizes a CRT is known as a film recorder. In such devices, a light beam from a CRT is controlled by a deflection system to scan across a stationary frame of photographic film to expose the film with an image. Typically, the images are generated by a computer for the production of slides, transparencies, photographs or the like. Alternatively, the images can comprise natural images acquired by a scanner or television camera for subsequent processing by a computer.
Known film recorders, such as those sold under the trademarks "SOLITAIRE" and "SAPPHIRE" by Management Graphics, Inc. of Minneapolis, Minn., U.S.A., typically include a CRT for producing a beam, a deflection system for scanning the beam across the face of the CRT, a system control and central processor unit ("CPU") for controlling the deflection system, and a camera (i.e., "film transport"). The latter component may include a film transport body, lens, lens mounting assembly, aperture plate, film plate, and film transport mechanism. The film may be provided in rolls or strips, or in individual sheets.
When used for very high quality imaging functions, such as in film recorders, conventional CRTs exhibit various limitations. One of the most significant limitations is in achieving the dynamic range that exists in photographic film. Typically, CRT images display a 100:1 contrast range between white and black. Film images and film exposure ranges can exceed 1000:1. The primary reason that CRTs do not reach this level is due to the optics of the glass-phosphor interface in the CRT.
Conventional CRTs which are viewed from the front are made of material which is transparent, so that the phosphor screen deposited on the interior of the faceplate can be seen. The material must also hold a vacuum. Glass is typically used for this purpose. However, since glass has an index of refraction different from that of air, internal reflections occur within the glass. These reflections result in an undesirable property known as halation, causing a halo effect around the point at which the electron beam impinges the phosphor. Because phosphor is typically white, the resultant visual flare reduces the overall contrast achievable in the image.
Another limitation in the use of CRTs for producing high quality images is the difficulty of achieving a blemish-free and uniformly bright phosphor screen over the entire image area. Even extremely small blemishes in the phosphor deposition, for example on the order of less than 0.001" can be enough to show as a defect in a recorded or scanned image.
Perfect screens are extremely difficult to fabricate for numerous reasons. One reason is that the phosphor layer must be very thin so that the electrons can penetrate it and the light generated by the phosphor can make it through the remainder of the phosphor and through the glass faceplate for viewing from outside the CRT. The uniform deposition of such a thin layer over a large image area is very difficult. Another significant factor in screen blemishes is the need for a metallic (e.g., aluminum), layer below the phosphor to serve as an anode. The metallic back layer must also be very thin. Known processes for providing such a back layer are very sensitive to contaminants.
It would be advantageous to provide a high resolution imaging display for use in film recorders and the like. Such a display should provide sharper spots and higher light output levels than conventional CRTs. It would be further advantageous to provide such a display that provides the image in a substantially centered relationship with an optical axis thereof, where the axis is substantially perpendicular to the plane in which the image resides. Such a construction should provide for the uniform viewing of an image for recordal by an image recordal or for scanning in an optical scanning apparatus.
The present invention provides a rearview CRT having the aforementioned advantages. Although rearview CRTs have been proposed in the past, they have not been successfully used for high quality image recorders. This is primarily due to the fact that the image viewing port was necessarily offset from the center of the CRT, since the electron gun portion of the CRT and the deflection yoke mounted to the CRT were always placed along the central CRT axis. An example of such a prior art design is provided in U.S. Pat. No. 3,526,800 for "Cathode Ray Tube Having Independent Front and Rear Displays." The CRT disclosed in the prior art patent was proposed for use in monitoring the image on the front of the CRT faceplate, and not to provide high quality image recording.